UTRAN (Universal Terrestrial Radio Access Network) is a term identifying the radio access network of a UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) and NodeBs i.e. radio base stations. The NodeBs communicate wirelessly with mobile user equipments (UEs) and the RNCs control the NodeBs. The RNCs are further connected to the Core Network (CN). Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further as illustrated in FIG. 1, the E-UTRAN consists of radio base stations (eNBs) 12, and the eNBs are interconnected and further connected to the Evolved Packet Core network (EPC) 14. E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP). FIG. 1 also shows UEs 10 in communication with the eNBs 12.
E-UTRAN is a pure packet data designed cellular system, in which transmissions of user data in uplink and downlink always take place via shared channels. Orthogonal Frequency Division Multiple (OFDM) technology is used in the downlink, whereas DFT (Discrete Fourier Transform) based pre-coded OFDM is used in uplink. As similar to HSPA (High Speed Packet Access) in the UTRAN, the UE monitors physical downlink control channels (PDCCH) in order to access UE dedicated user data on the physical downlink shared channel (PDSCH) and the network assigns uplink scheduling grants to the UE on demand basis for uplink transmission via the physical uplink control channel (PUCCH) and the physical uplink shared channel (PUSCH). Error detection is provided on transport blocks and control payloads through CRC, and HARQ operations ensure efficient re-transmissions.
FIG. 2 illustrates mapping of downlink common reference signals for one, two and four antenna ports.
OFDM is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence, in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks prior to being transmitted. A resource block is used both for uplink and downlink and is defined in both the time and the frequency domain. According to the E-UTRAN standard, a resource block size is 180 KHz (comprising of 12 sub-carriers each with 15 KHz carrier spacing) and 0.5 ms (time slot) in frequency and time domains, respectively. The transmission time interval (TTI) comprises of 2 time slots, which correspond to 1 ms length in time. The radio frame is 10 ms long. The overall uplink and downlink transmission bandwidth can be as small as 1.4 MHz and as large as 20 MHz.
Each time slot comprises of N number of OFDM symbols as illustrated in FIG. 2 and a cyclic prefix (CP) is appended to each OFDM symbol to enable the mitigation of inter-symbol interference. The number of OFDM symbols ‘N’ per slot depends upon the cyclic prefix in use. In small cells typically normal CP (around 5 μs) is used whereas in larger cells extended CP (around 17 μs) is used. Therefore when normal CP is used more OFDM symbols can be used in a slot.
Each element in the time frequency resource grid for antenna port p is called a resource element 20, which is uniquely identified by the index pair (k,l) in a slot represented; kth sub-carrier and lth OFDM symbol as shown in FIGS. 1 and 2.
FIG. 2 further illustrates the resource elements 24 used for downlink common reference signal transmission for extended CP as specified in E-UTRA technical specification 36.211. The notation Rp is used to denote a resource element used for common reference signal transmission on antenna port p. The common reference signals are also referred to as cell specific reference signal as opposed to UE specific reference signals which are also referred to as dedicated reference signals. As indicated in FIG. 2, some resource elements 20 denoted by crosses are not used for transmission on a specific antenna port.
Reference signals or pilot signals or pilot sequences or training sequences have similar meaning and are interchangeably used in literature. It is standardized set of signal sequences which are transmitted by the transmitter and are known a priori to the receiver. Their main objective is to assist the receiver to estimate the characteristics of radio channel, which especially varies over time due to user mobility.
The terms reference signal or reference symbol are also interchangeably used but they have similar meaning.
E-UTRAN utilizes multiple antennas techniques referred to as MIMO (Multiple Input Multiple Output) modes. Examples of such MIMO modes are precoding mode and beamforming mode. Precoding mode works with codebook-based transmission weights and utilizes common reference signal for channel estimation. A UE can determine a codebook index from the channel estimates and feed it back to the evolved NodeB (eNodeB). Beamforming mode works however with non-codebook based transmission weights, which requires in the downlink a dedicated reference signal that is precoded with the same transmission weights as the data and can be operated without any feedback of a codebook index. UE measurements for radio resource management, such as mobility management, are performed based on the common reference signals.
Regarding the dedicated reference signal pattern design for the beamforming mode, in one scenario where all the users in one cell are configured for beamforming mode, the shared channel is demodulated based on the dedicated reference signal. On the other hand, common channels such as broadcast channel or other common control channels such as PDCCH, which may be transmitted to all or a group of UEs, are typically sent over the entire cell. In other words they are not beamformed towards a particular UE. Hence, the UE needs to use common reference signals to demodulate all common channels. This means that the network has to transmit both dedicated and common reference signals in a cell supporting beamforming. The transmission of dedicated reference signals in addition to the common reference signals results in that the overall overhead of reference signals becomes quite high.
It is therefore desired to reduce this overhead when dedicated reference signals are transmitted to support beam-forming in a cell. One way is to reduce the common reference signal density, e.g. to only keep the common reference signals in the same OFDM symbols as the common control channel while still keeping the common control channel demodulation performance good enough. In other scenarios where there are users configured for the precoding mode within one cell, common reference signals are required for both common control channel demodulation and the shared channel demodulation. Then it is preferred that the common reference signal is spread among the whole subframe, i.e. higher density.
The common reference signals are also used by the UE to perform neighbour cell measurements, which are used for taking mobility decisions such as cell reselection and handover. The neighbour cell measurements include, e.g. reference signal received power (RSRP) and reference signal received quality (RSRQ). However, the number of available reference signals is reduced if e.g. beamforming is used. An insufficient number of reference signals may adversely impact the neighbour cell measurement performance.
Moreover, LTE supports MBSFN (Multi-Media Broadcast over a Single Frequency Network) operation on the same carrier as unicast traffic. This means that a subset of the subframes is allocated to MBSFN transmission from multiple cells. In such subframes the common reference signal transmission is reduced and the common reference signals available for measurements are only transmitted in the first symbol of each subframe. In other normal unicast subframes, the reference signals from antenna port 0 and 1 for mobility measurements are transmitted in four different separated subframes.
In the current LTE it is desired to use as many reference signals as possible for neighbour cell measurements in order to achieve a good mobility management. For this purpose the serving cell can provide neighbour cell configuration indicators to the UE on a dedicated channel. These indicators can be used to indicate whether or not the neighbour cells have the same configuration as the serving cell and whether there are any MBSFN subframes at all. In this way the terminal will be able to use as many common reference signals as possible when performing the measurements.